Safety Armor for Protection Against Gunfire and Process for Producing it

ABSTRACT

A safety armor for protection against gunfire, comprising a shield composed of an alloy steel which has a base carbon content of less than 0.3% by mass of carbon and has been enriched in strength-increasing elements including at least one of carbon and nitrogen by means of a thermochemical treatment in a surface zone extending from at least one outer surface of the shield, with the steel of the surface zone having an increased surface hardness as a result of a thermal treatment including at least one of hardening and tempering carried out after the thermochemical treatment, the steel is enriched to at least 0.5% by mass of carbon in the surface zone and has a minimum hardness of 55 HRC on the outer surface with the presence of carbides in the surface zone, with the shield having a silicon content of not more than 0.4% by mass both in the surface zone and in a lower hardness region adjoining the surface zone which has a carbon content and hardness less than the surface zone.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to DE 10 2005 023 952.8 filed May 20, 2005 and PCT/EP2006/004532 filed May 15, 2006.

FIELD OF THE INVENTION

The invention relates to safety armor for protection against gunfire, comprising a shield composed of an alloy steel which has a base carbon content of less than 0.3% by mass of carbon and has been enriched in strength-increasing elements such as carbon and/or nitrogen by means of a thermochemical treatment in a surface zone extending from at least one outer surface of the shield, with the steel having an increased surface hardness as a result of a thermal treatment such as hardening and/or tempering carried out after the thermochemical treatment. The invention further relates to a process for producing such an armor.

BACKGROUND OF THE INVENTION

Safety armor composed of fully hardened steels, hard manganese steels having manganese contents of from 12 to 14 percent by mass (% by mass) and at the same time carbon contents of from 1.2% by mass to 1.4% by mass or of plated, in particular roll-plated steel is known for armoring vehicles or other objects, e.g. guard posts. Examples of such commercial steels are the materials Ultrafort 6355 (from Edelstahl Witten-Krefeld, material number: 1.6355), Thyrodur® X and Z (from Edelstahl Witten-Krefeld, material numbers: 1.27XX), Dipro 50 and 60 M (from Dillinger Hütte GTS) and Secure 500 (from Thyssen Krupp Stahl AG), etc.

Armoring materials are generally subjected to a gunfire test to determine their safety characteristics, with the standards DIN EN 1522, edition: 1999-02 “Fenster, Türen, Abschlüsse-Durchschusshemmung-Anforderungen und Klassifizierung”; German version EN 1522: 1998 and DIN EN 1523, edition: 1999-02 “Fenster, Türen, Abschlüsse-Durchschusshemmung-Prüfverfahren”; German version EN 1523: 1998, in particular, being obligatory in Germany at present. In addition, these materials can also be tested in accordance with a safety standard applicable in a particular country, or in a manner which is not classified, using weapons other than those specified in the standards, e.g. the Kalashnikov in Russia.

The bullet penetration inhibition is reported in classes corresponding to the following table.

Type of weapon Mass of Test Bullet Class Caliber bullet (g) distance (m) velocity (m/s) B1 22 LR 2.6 10 360 B2 9 mm Luger 8.0 5 400 B3 .357 Magnum 10.2 5 430 B4 .44 Magnum 15.6 5 440 B5 M16 gun 4.0 10 950 5.56 × 45 B6 G1 gun 9.5 10 830 7.62 × 51 Soft core B7 G1 gun 9.8 10 820 7.62 × 51 Hard core

To achieve a high gunfire class, it is necessary for the materials used firstly to have a high surface hardness in order to fend off and destroy the bullet and secondly be sufficiently tough to dissipate the kinetic energy of the bullet without crack formation.

Safety armor of the type mentioned at the outset is known from RU 2 090 828 C1. This document describes steel armor which is to be used in body armor or shields and is, in contrast to the steels described above as commercially available, composed of a steel which has been enriched in strength-increasing elements as described, in particular carbon, in a surface zone by means of a thermochemical treatment. According to the patent, a specific type of steel having the following composition is used: C—0.20 to 0.27% by mass, Si—1.20 to 1.50% by mass, Mn—0.3 to 0.90% by mass, Ni—0.50 to 1.20% by mass, Cr—1.10 to 1.50% by mass, Mo 0.15 to 0.35% by mass, Fe— balance. It is particularly emphasized that the required high hardness is achieved by the alloy with silicon (Si) and the required high toughness is achieved by the alloy with nickel (Ni). However, the known safety armor has the disadvantages of a high, energy-consuming austenite formation temperature which for the material described is 920° C. and a heating rate of from 1.5 to 2.5° C./s which has to be adhered to strictly in the thermal treatment, since otherwise a ferritic-bainitic microstructure which no longer ensures the safety characteristics desired for the armor is obtained.

It is an object of the invention to provide safety armor of the type mentioned at the outset and a process for producing it, by means of which the desired safety characteristics for the armor, in particular a bullet penetration inhibition corresponding to the classes B3 and higher in accordance with the standards DIN EN 1522 and DIN EN 1523 can be ensured and a reduction in costs and a high degree of freedom in the technological implementation of the production process can be achieved.

SUMMARY OF THE INVENTION

This object is achieved by the steel having been enriched to at least 0.5% by mass of carbon, or in terms of the process being enriched, in the surface zone and in the presence, or in terms of the process with formation, of carbides on the outer surface having a minimum hardness of 55 HRC, which in terms of the process is obtained by the thermal treatment, with the shield having a silicon content of not more than 0.4% by mass both in the carbon-enriched surface zone and in a region which is only slightly enriched, if at all, with carbon and has a comparatively low hardness.

The invention is based on a recognition derived from various studies that an alloy with silicon and with nickel is not only unnecessary for producing safety armor in the gunfire classes B3 to B7 and for producing security when fired at by a military caliber weapon but, firstly, the necessary austenite formation temperature for a thermochemical or thermal treatment can be reduced and, secondly, an enrichment in carbon can be carried out in a much wider range than is known from the prior art, for example including overcarburization, as a result of the suppression of the influence of silicon as alloying element. This is not possible in the presence of silicon above a content of 0.4% by mass, since the silicon firstly restricts the austenite range of iron and reduces the stability of the iron carbides, in particular secondary iron carbides, formed according to the invention in the microstructure of the steel as far as decomposition to form graphite.

A particularly preferred material for the purposes of the invention is, from these points of view, a steel which has the following values for the chemical melt analysis: C—0.17 to 0.20% by mass, Si—0.20 to 0.30% by mass, Mn—1.15 to 1.30% by mass, P— not more than 0.030% by mass, S— not more than 0.030% by mass, Al—0.020 to 0.050% by mass, Cu— not more than 0.25% by mass, Ni— not more than 0.25% by mass, Cr—1.15 to 1.30% by mass, Ti—0.02 to 0.05% by mass, B—0.0015 to 0.004% by mass. This steel is case-hardening steel which is known under the material designation 1.7160 or 16 MnCr 5 BP and is commercially available.

This base material is brought to the required ballistic properties by means of the thermochemical treatment and the heat treatment. As thermochemical treatment, it is possible to carry out, in particular, a carburization, as a result of which either a hypoeutectoidic or preferably hypereutectoidic carbon content in the range of at least 0.5% by mass, but particularly preferably from 1.1% by mass to 3.5% by mass, is obtained on the side exposed to gunfire, so that, depending on the composition of the steel, formation of carbides, either, for example, cementite (Fe₃C), in particular secondary cementite, and/or particular carbides such as (Cr,Fe)₇C₃ is achieved.

On the side opposite of the side facing gunfire, it is likewise possible to set, with or without formation of carbides, a carbon content of from 0.5% by mass to 3.5% by mass, with the carbon content being able to be identical or different on the two opposite sides. As a result, the shield has carbon-enriched surface zones which extend from the two outer surfaces and have a comparatively high hardness and have identical or different carbon contents and identical or different hardnesses and which go over, as the carbon content decreases, into the region between the surface zones which is only slightly carbon-enriched, if at all, and has a comparatively low hardness.

It can, in particular, be provided that, corresponding to a selected depth (hardening depth) of the carbon-enriched surface zone(s) after hardening and tempering of the thermochemically treated steel, not, more than about 50%, preferably not more than about ⅓, of the thickness of the shield has essentially the original hardness of the steel or a slightly higher hardness and at least about 50%, preferably at least about ⅔, of the thickness of the shield has a higher hardness.

When two surface zones are present, these can, as a function of the threat class, be provided with different hardening depths and carbon contents. The purpose of this technical measure is to destroy or break up impinging hard ammunition on the side exposed to gunfire by means of a comparatively high hardening depth and substantial carbide enrichment and to prevent exit of the projectile on the rear side by means of an elastic and hard surface zone. In addition, fragmentational splintering-off of sharp-edged pieces of steel is prevented, as a result of which any additional backing remains protected against cutting.

In addition, it is also possible for the shield to have been only partly thermochemically treated in the region of the outer surfaces.

To achieve the desired properties, the carburization can be carried out one or more times. Another possibility is to carry out the thermochemical treatment as a carbonitradation, as a result of which not only carbide formation but also nitride formation occur in the vicinity of the surface, which is likewise advantageous for the purposes of the invention.

The formation of carbides can be influenced by multiple carburization at different carburization temperatures so that a gradated layer of fine and coarse carbides having a more or less strongly defined grain boundary is formed on at least one side. The abovementioned susceptibility of the surface zone to destruction by projectiles can be set in this way.

The thermal treatment, which comprises firstly a hardening by means of which the very high hardness in the outer zone is obtained by single or double hardening and the toughness in the core is obtained, makes it possible to achieve a desired hardness, in particular of at least 55 HRC, preferably more than 60 HRC, particularly preferably from 64 HRC to 67 HRC.

To achieve a good evenness of the surface of the shield, hardening can be carried out, in particular, in a chill roll unit, i.e. with forcing of the shape and application of pressure. It is also advantageous in process engineering terms that an increased hydrostatic pressure decreases the austenite formation temperature. To set the final hardness and toughness, both in the carbon-enriched surface zone and in the region of comparatively low hardness which has been enriched only slightly, if at all, with carbon, tempering can preferably be carried out after hardening.

Apart from the steel which has been mentioned above as particularly preferred, it is also possible for the steel used to be a low- or high-alloy steel containing chromium, in particular chromium together with manganese and/or molybdenum, e.g. 25 CrMo 4, 17 CrNiMo6, 13 CrMo 4 5, 20 MnCr5 or X 19 NiCrMo 4.

The presence of chromium in the amount characteristic of these steels reduces the austenite formation temperature, promotes carbide formation, reduces the critical cooling rate for hardening and increases the hardenability. The chromium content of the steel should be not greater than about 1.6% by mass and preferably be in the range from 1.15% by mass to 1.30% by mass.

The presence of molybdenum increases the hardness and reduces the embrittlement on tempering. The molybdenum contents preferred for this purpose are in the range from 0.20% by mass to 0.40% by mass. Furthermore, molybdenum is capable of forming carbides (MoC, MoC₂, mixed carbides with iron) as provided according to the invention. Molybdenum also advantageously promotes the increase in the penetration depth of carbon in the thermochemical treatment and thus leads to an increase in the carbon content in the surface zone.

In this context, it may be pointed out that the presence of nickel, although not considered essential for the purposes of the invention, in an amount of more than 1% by mass is not ruled out according to the invention and the steel can therefore also be a high-alloy, in particular nickel-containing steel such as the abovementioned X 19 NiCrMo 4.

For the purposes of the invention, it is considered to be particularly advantageous for the steel to be a low-alloy, predominantly manganese-containing steel such as 16 MnCr 5, 16 MnCrS 5, 20 MnCr 5, 21 MnCr or 20 MnCrS 5. The presence of manganese in the amount characteristic of these steels reduces the austenite formation temperature and the critical cooling rate during hardening and increases the hardenability. In addition, manganese forms manganese sulfides which compensate for any harmful effect of sulfur. It is therefore possible for the sulfur concentrations present in the material to be higher than for the steel mentioned above as particularly preferred. As a result of manganese increasing the carbon activity in iron, decomposition of iron carbides is inhibited, in contrast to the circumstances when silicon is present. Manganese is also known to act as a deoxidant. According to the invention, the manganese content of the steel should be, in particular, greater than about 0.8% by mass, preferably greater than 1.0% by mass, but less than about 2.5% by mass, preferably less than 2.0% by mass, and very particularly preferably in the range from 1.15% by mass to 1.30% by mass.

Up to a carbon content of about 1.6% by mass, these manganese steels are thus perlitic manganese steels, i.e. not the hard manganese steels which are used in a known fashion for producing safety armor and have a very much higher manganese content. However, the carbon enrichment which takes place according to the invention and can lead to carbon contents which are unusually high for a carburization can initially result in austenitic microstructure formation which comes close to that in hard manganese steels in the surface zone of the shield.

The shield of the safety armor can, in particular, be configured in the form of a plate, i.e. having at least two, in particular essentially parallel, outer surfaces which have larger dimensions than the edge faces present. The plate size (length and width) depends on the particular application and its maximum dimensions are determined by the technological circumstances in the working of metal sheets. The shield should preferably have a minimum thickness of about 3.0 mm; a maximum thickness should be in the range from about 10.00 mm to 25.0 mm.

It can be considered to be advantageous that in the safety armor of the invention the shield is joined to a substrate which has, in particular, a high strength, impact toughness, tear resistance, chemical resistance, flame resistance and/or is self-extinguishing, e.g. a protective woven fabric comprising para-aramid fibers. Such protective woven fabrics are known, for example, under the name Kevlar®.

Further advantageous embodiments of the invention may be found in the subordinate claims and in the following description.

BRIEF DESCRIPTION OF THE DRAWING

The invention is illustrated below with the aid of an example and reference to the accompanying drawing. In the drawing:

FIG. 1 shows a cross section through safety armor according to the invention for protection against gunfire, and

FIG. 2 shows a greatly enlarged polished section of a microstructure characteristic of safety armor according to the invention.

DETAILED DESCRIPTION

As can be seen from FIG. 1, safety armor according to the invention for protection against gunfire comprises a shield 1. This shield 1 comprises an alloy steel which has a base carbon content of less than 0.3% by mass of carbon, as is indicated schematically in the right-hand part of the curve in the graph present in the figure, which shows the change in concentration of carbon (C) over the cross section (thickness d) of the shield 1. The corresponding region of the shield is denoted by the reference letter B. The carbon content is thus in a concentration range which is characteristic of case-hardening steels.

The steel has the following values of the chemical melt analysis: C—0.17 to 0.20% by mass, Si—0.20 to 0.30% by mass, Mn—1.15 to 1.30% by mass, P— not more than 0.030% by mass, S— not more than 0.030% by mass, Al—0.020 to 0.050% by mass, Cu— not more than 0.25% by mass, Ni— not more than 0.25% by mass, Cr—1.15 to 1.30% by mass, Ti—0.02 to 0.05% by mass, B—0.0015 to 0.004% by mass.

As a result of thermochemical treatment, a surface zone R extending from the outer surface D1 of the shield 1, which represents the side exposed to gunfire, is enriched with strength-increasing elements, viz. carbon, which is indicated schematically by the left-hand part of the curve in the graph. The shield 1 is thus carburized, in particular supercarburized, in the surface zone R to enrich it in carbon.

The thermochemical treatment for carburization can be carried out at a temperature in the range from 900° C. to 1040° C. for a treatment time in the range from 30 to 720 minutes, in particular by means of a gaseous medium, e.g. in a propane-enriched endothermic atmosphere or at a low pressure by means of ethyne. If desired, the carbon concentration can be increased to a value of at least 0.8% by mass, in particular from >1.1% by mass to 3.5% by mass, in the surface zone R by multiple repetition of the thermochemical treatment.

As a result of a thermal treatment, e.g. hardening and/or tempering, carried out after the thermochemical treatment, the steel has an increased surface hardness determined by the Rockwell method (Rockwell C) on the outer surface D1. In particular, a hardness in the range from about 60 to 67 HRC is set on the outer surface D1 of the shield 1.

The thermal treatment encompasses austenite formation, preferably at a temperature in the range from 800° C. to 880° C., with subsequent quenching, in particular by means of oil. Quenching is preferably carried out in a chill roll unit, i.e. with forcing of the shape and application of pressure.

Austenite formation and quenching can, if necessary, be carried out as double hardening, with the austenite formation temperature being matched to the region B of the steel which is enriched with carbon to only a small extent, if any, in a first hardening step and the austenite formation temperature is matched to the carbon-enriched region R of the steel in a second hardening step. As a result, very fine needles of martensite are formed as hardening microstructure and refining of the grain size is also achieved in the region B which is enriched to only a small extent with carbon by the annealing action which occurs.

The thermal treatment which follows the thermochemical treatment encompasses tempering, in particular for a tempering time in the range up to 3 hours at a temperature in the range up to 300° C. The hardness in the uncarburized or only slightly carburized region B of the shield 1 can subsequently be, in particular, in the range from about 35 to 47 HRC.

The sketch in FIG. 1 indicates that, corresponding to a selected depth t of the carbon-enriched surface zone R after hardening and tempering of the steel, less than 50% of the thickness D of the shield 1 has essentially the original hardness of the steel or a slightly higher hard ness and at least about 50% of the thickness D of the shield 1 has a higher hardness. This thickness D of the shield 1 can be, in particular, in the range from about 3.0 mm to 10 mm, for example 7.5 mm.

As can also be seen from FIG. 1, it is provided, in the depicted embodiment of the invention, that the shield 1 is joined to a substrate 2 which has, in particular, a high strength, impact toughness, tear resistance, chemical resistance, flame resistance and/or is self-extinguishing, for example to a protective woven fabric comprising para-aramid fibers, and is located, in particular, on the side facing away from the direction of gunfire.

The electron micrograph of a polished section of the microstructure G on the outer surface D1 of the carbon-enriched surface zone R shown in FIG. 2 shows that a microstructure G typical of safety armor according to the invention comprises a matrix M of a tempered mixed microstructure which contains martensite and a small proportion of residual austenite and/or an intermediate microstructure such as bainite. Further more, it can be seen that the carbon-enriched surface zone hash preferably spherical and lamellar embedded particles E in the microstructure G. These embedded particles E can be identified chemically as nitrides or carbides, in particular secondary iron carbides or else particular carbides such as mixed carbides of chromium and/or molybdenum and are, in addition to the martensite, responsible for the high surface hardness of the outer surface D1.

Determination of the bullet impact properties of the safety armor of the invention in accordance with the standards DIN EN 1522 and DIN EN 1523 showed that the shield 1 (without woven fabric substrate 2) in each case had a bullet penetration inhibition corresponding to class B3 and higher, with there being a tendency for the higher classes to be associated with a higher carbon content, double hardening and quenching in the chill roll unit and both the gunfire class B7 and security against gunfire when using military caliber weapons being achieved.

The invention is not restricted to the example presented, but encompasses all embodiments which have the same effect within the scope of the invention. Thus, it has already been pointed out that the composition of the steel of the shield 1 can deviate from the melt analysis presented by way of example.

In this context, it may be emphasized that even use of an alloy steel which has a base carbon content of less than 0.3% by mass of carbon and a silicon content of not more than 0.4% by mass and is enriched in strength-increasing elements such as carbon and/or nitrogen in a surface zone by means of a thermochemical treatment is also of importance according to the invention as material for producing safety armor for protection against gunfire, which can be produced, in particular, by the above-described process. Further rules which relate to alloy compositions for the shield 1 which are regarded as particularly advantageous, e.g. the content of manganese, chromium, molybdenum and nickel, have been mentioned above.

Furthermore, a person skilled in the art will be able to envisage other advantageous features or technical measures for the structure of the safety armor of the invention. It has already been mentioned that the shield 1 can have carbon-enriched surface zones R of comparatively high hardness which extend from its two outer surfaces D1, D2 and can have identical or different carbon contents and accordingly identical or different hardnesses. The region B which is enriched with carbon to only a slight extent, if at all, and has a comparatively lower hardness would then be located between these two surface zones R located under the outer surfaces D1, D2.

The scope of the invention also encompasses the case where the two outer surfaces D1, D2 of the shield 1 of the safety armor of the invention do not run parallel to one another but instead at a slight angle to one another, which may sometimes enable improved protection against gunfire to be achieved.

While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims. 

1. A safety armor for protection against gunfire, comprising a shield a composed of an alloy steel which has a base carbon content of less than 0.3% by mass of carbon and has been enriched in strength-increasing elements including at least one of carbon and nitrogen by means of a thermochemical treatment in a surface zone extending from at least one outer surface of the shield, with the steel of the surface zone having an increased surface hardness as a result of a thermal treatment including at least one of hardening and tempering carried out after the thermochemical treatment, the steel following the thermochemical and thermal treatment is enriched to at least 0.5% by mass of carbon in the surface zone and has a minimum hardness of 55 HRC on the outer surface with the presence of carbides in the surface zone, with the shield having a silicon content of not more than 0.4% by mass both in the surface zone and in a lower hardness region adjoining the surface zone which has a carbon content and hardness less than the surface zone.
 2. The safety armor as claimed in claim 1, characterized in that the shield has two surface zones which extend from a first and a second of the outer surfaces and have a comparatively high hardness and have identical or different carbon contents and identical or different hardnesses and which, as the carbon content decreases, blend into the lower hardness region between the surface zones.
 3. The safety armor as claimed in claim 1 wherein the chemical treatment of the shield is a carburization, in particular, in the surface zone(s) to enrich the surface zone in carbon.
 4. The safety armor as claimed in claim 1 wherein the thermochemical treatment of shield is is carbonitrization in the surface zone to enrich the surface zone in carbon.
 5. The safety armor as claimed in claim 1 wherein the alloy steel is a low- or high-alloy steel containing chromium, together with at least one of manganese and molybdenum.
 6. The safety armor as claimed in claim 1 wherein the alloy steel is a low-alloy, predominantly manganese-containing steel.
 7. The safety armor as claimed in claim 1 wherein the manganese content of the alloy steel is greater than about 0.8% by mass, but less than about 2.5% by mass.
 8. The safety armor as claimed in any of claim 1 wherein the chromium content of the alloy steel is not greater than about 1.6% by mass.
 9. The safety armor as claimed in claim 1 wherein the alloy steel has the following values of the chemical melt analysis: C—0.17 to 0.20% by mass, Si—0.20 to 0.30% by mass, Mn—1.15 to 1.30% by mass, P— not more than 0.030% by mass, S— not more than 0.030% by mass, Al—0.020 to 0.050% by mass, Cu— not more than 0.25% by mass, Ni— not more than 0.25% by mass, Cr—1.15 to 1.30% by mass, Ti—0.02 to 0.05% by mass; B—0.0015 to 0.004% by mass.
 10. The safety armor as claimed in claim 1 wherein the alloy steel is a high-alloy, in particular nickel-containing steel X 19 NiCrMo
 4. 11. The safety armor as claimed in claim 1 wherein the alloy steel is enriched with carbon to at least 0.8% by mass, in the surface zone.
 12. The safety armor as claimed in claim 1 wherein the alloy steel has a hardness of more than 60 HRC in the surface zones.
 13. The safety armor as claimed in claim 1 wherein after the thermal treatment and thermochemical treatment, the surface zone is at least about 50% of the thickness of the shield.
 14. The safety armor as claimed in claim 1 wherein after the thermal treatment and thermochemical treatment, the surface zone is at least about ⅔ of the thickness of the shield.
 15. The safety armor as claimed in claim 1 wherein the surface zone following the thermochemical and thermal treatment comprises a matrix with a microstructure which contains martensite and at least one of a small proportion of residual austenite and intermediate microstructures including bainite.
 16. The safety armor as claimed in claim 1 wherein the surface zone following the thermal and thermochemical treatments comprise(s) carbides, including one or more of secondary iron carbides and mixed carbides of chromium and mixed carbides of molybdenum, and nitrides.
 17. The safety armor as claimed in claim 1 wherein the shield is configured in the form of a plate having at least two, of the outer surfaces which are generally parallel which have larger dimensions than edge faces formed by the plate.
 18. The safety armor as claimed in claim 1 wherein the shield has a minimum thickness of about 3.0 mm.
 19. The safety armor as claimed in claim 1 wherein the shield has a maximum thickness in the range from about 10.0 mm to 25.0 mm.
 20. The safety armor as claimed in claim 1 wherein the shield has been only partly thermochemically treated in the region of one or more of the surface zones.
 21. The safety armor as claimed in claim 1 wherein the shield is joined to a substrate which has one or more characteristics including high strength, impact toughness, tear resistance, chemical resistance, flame resistance and self-extinguishing.
 22. A process for producing safety armor for protection against gunfire, in which a shield composed of an alloy steel which has a base carbon content of less than 0.3% by mass of carbon comprising the steps of enriching the shield in strength-increasing elements including one or more of carbon and nitrogen by means of a thermochemical treatment in a surface zone extending from at least one outer surface of the shield and a subsequent thermal treatment including one or more of hardening and tempering, the alloy steel having a silicon content of not more than 0.4% by mass is enriched to at least 0.5% by mass of carbon in the surface zone by means of the thermochemical treatment and in that a minimum hardness of 55 HRC is set on the outer surface by means of the thermal treatment.
 23. The process as claimed in claim 22 wherein the thermochemical treatment is carried out as a carburization at a temperature in the range from 900° C. to 1040° C. for a treatment time in the range from 30 to 720 minutes, a gaseous medium.
 24. The process as claimed in claim 22 wherein the thermochemical treatment is carried out as a carbonitridation.
 25. The process as claimed in claim 22 wherein the shield is enriched with carbon in a pair of the surface zones extending from two of the outer surfaces of the shield with the surface zones having a relatively high hardness being configured with identical or different carbon contents and identical or different hardnesses and blending, as the carbon content decreases, into a lower hardness region, between the surface zones, of comparatively low hardness which is enriched at most only slightly with carbon.
 26. The process as claimed in claim 22 wherein the alloy steel is enriched with carbon to at least 0.8% by mass by the thermochemical treatment in the surface zones.
 27. The process as claimed in claim 22 wherein following thermal treatment the surface zone comprises austenite formation, and is carried out a temperature in the range from 800° C. to 880° C., with subsequent quenching.
 28. The process as claimed in claim 27 wherein the quenching is carried out in a chill roll unit with forcing of the shape of the shield by applying pressure.
 29. The process as claimed in claim 27 wherein austenite formation and subsequent quenching is carried out as double hardening, with the austenite formation temperature being matched to the lower hardness region of the steel in a first hardening step and the austenite formation temperature is matched to the surface zone of the steel in a second hardening step.
 30. The process as claimed in claim 27 wherein a hardness of from about 60 to 67 HRC is set on the one or more outer surface(s) of the shield by means of the austenite formation and the subsequent quenching.
 31. The process as claimed in claim 22 wherein the subsequent thermal treatment comprises tempering, in particular for a tempering time in the range up to 3 hours at a temperature in the range up to 300° C.
 32. The process as claimed in claim 22 wherein a hardness of from about 35 to 47 HRC is set in the lower hardness region of the shield.
 33. A process for protecting against gunfire comprising the steps of providing an alloy steel which has a base carbon content of less than 0.3% by mass of carbon enriching the steel in strength-increasing elements of one or more of carbon and nitrogen by means of a thermochemical treatment in a surface zone, the steel having a lower hardness zone blending into the surface zone.
 34. The process as claimed in claim 33 wherein the carbon content set in the surface zone is at least 0.5% by mass of carbon following the enriching stem.
 35. The process as claimed in claim 33 wherein the carbon content set in the surface zone has a hypereutectoidic concentration, in particular a concentration of from 1.1% by mass to 3.5% by mass following the enrichment step.
 36. The process as claimed in claim 33 wherein the alloy steel provided is a low or high-alloy steel containing chromium, together with one or more of manganese and molybdenum.
 37. The process as claimed in any claim 33 wherein the steel is a low-alloy, predominantly manganese-containing steel such as 16 MnCr 5, 16 MnCr 5 BP, 16 MnCrS 5, 20 MnCr 5, 21 MnCr 5 or 20 MnCrS
 5. 38. The process as claimed in claim 33 wherein the manganese content of the steel provided is greater than about 0.8% by mass but less than about 2.5% by mass.
 39. The process as claimed in claim 33 wherein the chromium content of the steel provided is not greater than about 1.6% by mass.
 40. The process as claimed in claim 33 wherein the steel has the following values of the chemical melt analysis: C—0.17 to 0.20% by mass, Si—0.20 to 0.30% by mass, Mn—1.15 to 1.30% by mass, P— not more than 0.030% by mass, S— not more than 0.030% by mass, Al—0.020 to 0.050% by mass, Cu— not more than 0.25% by mass, Ni— not more than 0.25% by mass, Cr—1.15 to 1.30% by mass, Ti—0.02 to 0.05% by mass, B—0.0015 to 0.004% by mass.
 41. The process as claimed in claim 33 wherein the steel provided is a high-alloy, nickel-containing steel. 